1. Field of the Invention
This invention relates to devices and systems for mixing air and fuel for operation of internal combustion engines, and is directed more particularly to a flow proportioning mixer and system for use with internal combustion engines using gaseous fuel.
2. Description of the Prior Art
Concerns relative to fuel supplies and air quality have led to serious consideration of natural gas as an alternative to gasoline for use in internal combustion engines, and particularly in vehicular internal combustion engines. Some advantages of natural gas include little or no particulate emissions, reduced toxic emissions, and high octane rating which lends itself to higher engine compression ratios with consequent improvement in efficiency.
To convert vehicular gasoline engines to operate on natural gas, it is critical to control the flow of air and gaseous fuel such that an optimum fuel-air mixture is maintained at all times during operation of the engines. Normal operating conditions for internal combustion engines used in vehicles include fast transients on speed and power output, pronounced swings in air and/or fuel temperatures, and variations in fuel composition, all of which increase the difficulty of maintaining fuel-air mixtures within desired tolerances.
In general, the function of a mixer is to meter the gas flow rate according to the air flow rate, such that the two flow rates are maintained in proportion as the flow rates change.
There are essentially two known types of mixers designed to achieve the maintenance of proportioned flow rates. A first of the two general types utilizes a pressure differential across an orifice, or nozzle, in the air stream to (1) induce gas flow, or (2) control an orifice size in the gas stream. A second type involves measurement of air flow rate with a flow transducer, processing the air flow data in a computer to determine the required gas flow, and controlling a fuel injector to deliver the desired fuel flow.
Mixers of the first general type, employing pressure differential in the air stream to control gas flow, achieve the objective of maintaining a constant fuel-air ratio in a number of ways. One known type, in use for many years in industrial gas engines, applies a pressure differential generated by air flow on a diaphragm to modulate a gas valve. Some disadvantages to this type mixer are (1) the diaphragm is always in motion when engine power is varied continuously, as in normal vehicle operation, causing wear and consequently affecting the reliability of the mixer; (2) the response tends to be slow in the case of fast transient operation, leading to momentary deviations from the desired fuel-air flow ratio; and (3) the fuel-air flow ratio is affected by a number of operational parameters, making it difficult to obtain a constant fuel-air ratio over a wide range of engine power. Such disadvantages have not been important factors in industrial gas engines which operate at steady power for a majority of the time, but have a pronounced detrimental effect on emissions when applied to vehicular engines.
Another of the first general type of mixer is the venturi mixer, which utilizes Bernoulli's law to induce gas flow into the throat of a venturi in which there is air flow. The rate of gas flow is proportional to the rate of air flow and, in theory, should remain so, as long as the supply pressures of air and gas are equal and the temperatures of the air and gas remain constant. However, when the fuel temperature, or the air temperature, is changed, the fuel-air ratio will deviate from the selected value. Further, when the composition of the natural gas is changed, as from one supplier to another, the selected fuel-air ratio should be changed to achieve effective reduction of emissions. In short, the known venturi type mixers lack the flexibility required to vary the fuel-air ratio, as is needed in vehicular applications.
However, it remains true that venturi-type mixers are simple and reliable. It would be of great benefit to the motoring public and industry to have available a venturi-type mixer adapted to automatically react to changes in fuel composition and to changes in temperature of the gas and/or air, and, further, to be responsive to the continuously varying power demands of vehicular engines.